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The minimalist movement of the last few years has gotten runners focused on their feet. Many now know what research has shown: Foot-strike patterns are variable, and the specific interaction between the foot and the ground is influenced by multiple factors like speed, surface and footwear. So it shouldn't come as a surprise that there's a general lack of consensus on just what one's feet should be doing while one is running. The question arises: Does foot-strike type even matter?
Each foot comes into contact with the ground 80 to 100 times per minute on average--this translates to a stride rate or cadence of 160 to 200 steps per minute, with considerable variation from person to person and at different running speeds. Every time you land, your foot impacts the ground with a certain amount of force, which is counteracted by an equal and opposite amount of force applied by the ground to your foot. This equal and opposite force is known as the ground reaction force (GRF). The GRF comes in a number of components, typically broken into anterior-posterior (along the direction you're traveling), horizontal (side to side) and vertical (straight up and down). Of these, the vertical GRF is the greatest in magnitude and is the one we'll focus on here.

The graph below depicts a hypothetical vertical GRF curve for a heel-striking barefoot runner.

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On the vertical (Y) axis is the value of the vertical GRF as a function of body weight. You can think of the vertical GRF as the amount of force that each collision with the ground exerts directly upward through your foot. The horizontal (X) axis shows time in milliseconds. The amount of time each foot is in contact with the ground varies among people and at different running speeds, but 300 milliseconds (about one-third of a second) is a reasonable number for a typical recreational runner. The curve depicted on the graph shows how vertical GRF changes from the point of initial contact of the foot with the ground (time 0) to the point where the foot leaves the ground on toe-off (about 300 ms).

What you'll notice in this graph is that there are two distinct force peaks. First, the impact peak, sometimes called the passive peak, is the initial force applied by the ground to the foot and lower leg at initial heel contact (remember, this graph is for a heel-toe runner). Full body weight isn't being applied at this point, so the impact peak is basically a function of the weight of some portion of the foot and lower leg hitting the ground. Second, the active peak is a function of the force applied by the foot and supported body weight during roughly midstance.

Foot Strike

Notice that it's larger and longer in duration than the impact peak; this is the typical pattern.

In the graph, vertical loading rate is the slope of the line from initial contact to the impact peak (in practice, it's usually measured in the region from 20 to 80 percent between these points). The loading rate simply represents how quickly the impact force is applied--a steeper slope means a more rapid collision. A more gentle slope would indicate that force application during impact is being spread out over a longer period of time. A reasonable analogy would be punching the wall with your bare fist--your fist comes to a very rapid stop and the force is applied quickly. This is not a good thing for your fist! On the other hand, if you put on a boxing glove, you could hit the wall really hard, but the cushion in the glove would slow the impact and distribute the force over a longer period of time. Consequently, it wouldn't hurt as much, and would likely not damage the bones in your hand. The same logic applies to crumple zones in a car--in a crash the crumple zone slows force application to protect the driver and passengers from the sudden shock of an immediate impact.

Because loading rate increases dramatically, and along with it the risk of an injury like a stress fracture, heel striking while running barefoot is probably not a sustainable option for the long term. Thus, if you're going to run in a heel-toe style, putting cushion under the heel makes sense--it will slow impact and decrease the loading rate by spreading impact force over a longer period of time. This is a good thing for the heel bone (the calcaneus), and is why many barefoot runners and people who prefer barefoot-style shoes tend to move away from a pronounced heel strike, particularly on hard surfaces (it may not happen instantly, but most will likely adapt with time). The fatty heel pad doesn't provide enough protection to absorb continual hard impact during barefoot heel-toe running, so your options are either to alter your form or provide some type of protection for your heel.


Now, let's take a look at what usually happens if a runner were to switch to a barefoot forefoot strike.

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What you'll note in the graph above is that the distinct impact peak (sometimes referred to as the "impact transient") is now gone. By eliminating the heel strike, the runner has eliminated the impact peak, and the initial slope of the rising force is now lower; loading rate has decreased. The reason for these changes is that, instead of absorbing impact via a collision between the heel and the ground, the runner now uses the springiness of the arch of the foot and the Achilles tendon/calf muscle complex to cushion the blow.

Physical therapist Jay Dicharry, of the UVA Center for Endurance Sport, uses an analogy from the automotive world to illustrate the difference between heel striking and forefoot striking. If a standard car is like a heel-striker, then a monster truck is like a forefoot striker, Dicharry says. "Imagine that your car has one shock absorber per axle/per wheel. That shock absorber is there to dissipate a certain amount of stress. Now consider those giant Bigfoot-like monster trucks. These trucks have all of these extra shocks and struts that dissipate the huge amounts of force that those chassis have to endure. If you have more shocks, the vehicle can dissipate more load a little bit more evenly and a little bit easier. There's less stress to the system.

"So when you run," continues Dicharry, "if you land with a very prominent heel strike, the ankle doesn't really give a whole lot. It's a stiff system. So all of the compliance has to be shifted to the knee. And if you do land on the forefoot, what you essentially wind up doing is adding a shock absorber to your chassis. It allows you another joint through which you can dissipate some force." What a forefoot landing also does, though, is add stress to the foot and ankle that a heel-striker who is attempting to alter foot strike is likely not adapted to, which could explain the incidence of transition injuries among fledgling barefoot and minimalist runners.

Even though differences in patterns of force application among the different foot-strike types do seem to be fairly consistent, Dicharry is careful to point out that foot strike is just one part of overall running form, and that some people do exhibit high impact with a forefoot strike, while others exhibit low impact with a heel strike. "Just because you put somebody on the forefoot doesn't necessarily make everything beautiful, but it is a tool you have available to make a difference."

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It's important to highlight that inserting cushioning into shoes to manage impact does work--heel-toe running in a shoe with a cushioned heel does reduce impact loading rate. In fact, Daniel Lieberman's barefoot running study showed that average impact loading rate wasn't significantly different between shod heel-toe runners and barefoot forefoot strikers. As always, it's important to note that variation among individuals was considerable. Lieber-man reported that for "the majority of barefoot FFS [forefoot strike] runners, rates of loading were approximately half those of shod RFS [rearfoot strike] runners."

But even if the cushioned heel reduces impact loading rate so that it's on average equivalent to a barefoot forefoot strike, the foot and leg are generally not positioned the same way in the two conditions. As a result, external forces of similar magnitude can be applied to the body in different ways, and they might be ways that the human body hasn't evolved to handle as well. Unfortunately, it's quite difficult in practice to know just how the impact reaction force applied by the ground might manifest itself at the level of individual tissues and joints. Though scientists can and do use complex biomechanical models to estimate these forces, to obtain true measures of forces inside or around the ankle, knee or hip would require implanting sensors into the legs of actual runners--good luck trying to find volunteers for that study!

Why do impact characteristics even matter? There's much debate regarding the potential likelihood of a causal relationship between impact loading rate and running injuries. For example, several studies have linked higher vertical impact loading rates, but not impact peaks, to injuries like lower extremity stress fractures and plantar fasciitis. Additionally, a 2012 study by Adam Daoud, a former student in Lieberman's lab, shows that Harvard cross country runners "who habitually rearfoot strike had approximately twice the rate of repetitive stress injuries than individuals who habitually forefoot strike." Conversely, Benno Nigg writes in his book, Biomechanics of Sport Shoes, that based on his research, he thinks that "impact forces are not responsible for the development of specific injuries." He points out that internal joint forces at impact are much smaller than they are during the midstance phase of the gait cycle when the entire weight of the body bears down on the lower limb. Nigg concludes that "from an impact perspective, the 'cushioning' aspect of shoes needs not take injuries into account." In other words, if the goal is to design shoes to minimize injury risk, they should be designed to manage forces at midstance, not at impact.

Other experts take a more balanced viewpoint and recognize that both elevated impact and midstance forces could be problematic, likely for different types of injuries. For example, based upon his combined clinical and research experience, Dicharry thinks that reducing loading rate is critical.

"Imagine running 50 miles a week. Think of the amount of wear and tear that occurs on the body," he wrote on his UVA Endurosport blog. "Now imagine running 50 miles a week with a gait pattern that causes the mechanical loading of the body to occur less quickly. Decreasing the loading rate applied to tissues will minimize tissue stress to the runner, minimizing the effects of the microtrauma of endurance training." But, like Nigg, Dicharry points out that midstance forces are indeed significantly higher and are also extremely important to consider when it comes to injury risk.

Given our current state of knowledge, forces at both initial impact and at midstance should be considered when it comes to designing running shoes and making recommendations about proper form. A single-minded focus on just one or the other is likely misguided.

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Adapated with permission from Tread Lightly, which was published in May by Skyhorse Publishing.

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